The invention relates to problems involving changeovers from audio information derived from one source to audio information derived from another source, during the audio playback incident to the projection of motion-picture sound film. An example of this type of problem would be a motion-picture sound-film recording of action accompanied by spoken dialogue, with music to be edited-in during certain time intervals, for instance during time intervals when action is occurring without spoken dialogue. Conventionally, this sort of changeover effect is implemented using editing-studio re-recording techniques or the like. In such a situation, after music has been edited-in onto the motion-picture sound film, the audio track thereon will no longer contain the original information; i.e., the music, or the like, will supplant the original audio recording, e.g., will supplant the recording of background dialogue not related to the filmed action.